Lymphokines have been released in vivo into the circulation of some strains of inbred mice after intravenous sensitization with 300 microgram cell walls of Mycobacterium bovis strain BCG and intravenous challenge three weeks later with 50 mg old tuberculin (OT). Different strains of inbred mice varied in their capacity to release migration inhibitory factor (MIF) and type II interferon (IF II) in vivo, with the capacity to release these two lymphokines associated at least in part with the H-2 region. Thus, the low-responder strains were of the H-2k or H-2q types, while the high-responder strains were H-2a, H-2b, H-2d, or H-2s. The low-responder and high responder strains released the maximum amounts of the two lymphokines at the same time after intravenous challenge. This pattern of if vivo release was different for other lymphokines released in vivo under the same experimental conditions, i.e., chemotactic factor, mitogenic factor, and skin-reactive factor. The resistance of the various inbred strains to sublethal challenge, as measured by quantitative assay of the number of yeast cells per mouse kidney, varied according to the particular strain. This resistance is now being studied in relationship to that strain's capacity to release each lymphokine in vivo into the circulation after challenge with C. albicans.